text: The PEP carboxylase enzyme is present in plants and some types of bacteria, but not in fungi or animals (including humans). The genes vary between organisms, but are strictly conserved around the active and allosteric sites discussed in the mechanism and regulation sections. Tertiary structure of the enzyme is also conserved.
  The crystal structure of PEP carboxylase in multiple organisms, including Zea mays (maize), and Escherichia coli has been determined. The overall enzyme exists as a dimer-of-dimers: two identical subunits closely interact to form a dimer through salt bridges between arginine (R438 - exact positions may vary depending on the origin of the gene) and glutamic acid (E433) residues. This dimer assembles (more loosely) with another of its kind to form the four subunit complex. The monomer subunits are mainly composed of alpha helices (65%), and have a mass of 106kDa each. The sequence length is about 966 amino acids.
  The enzyme active site is not completely characterized. It includes a conserved aspartic acid (D564) and a glutamic acid (E566) residue that non-covalently bind a divalent metal cofactor ion through the carboxylic acid functional groups. This metal ion can be magnesium, manganese or cobalt depending on the organism, and its role is to coordinate the phosphoenolpyruvate molecule as well as the reaction intermediates. A histidine (H138) residue at the active site is believed to facilitate proton transfer during the catalytic mechanism.